Dumb Questions For the Chemist

The first half of the story is something called a saturated solution, which is in this case a carbonated beverage or soda pop. All of the bubbles in a soda pop that make you burp come from carbon dioxide gas that is dissolved into the soda solution. While the soda is in the bottle, the gas is kept in solution by the pressurized conditions inside the bottle. But after you pour some soda into a glass, the gas bubbles stay trapped in the solution by the surface tension of the water. No wonder soda makes you burp— those gas bubbles are just sitting in there waiting to escape!

The second half of the story is something called a nucleation site. Looking at a piece of Mentos® candy, you may think it is very smooth. But if you were to look under a microscope, you would see tiny bumps coating the entire surface of the candy. Each tiny bump acts like a nucleation site, a place where this physical reaction can get a kick start. Each tiny nucleation site becomes a place where a bubble of carbon dioxide gas can form and escape the solution. Multiply that by all of the tiny bumps on a Mentos® and you have yourself a geyser!

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You might think that there is some ingredient in a Mentos® candy that is causing a chemical reaction with the soda, like the way baking soda reacts with vinegar. But this is not a chemical reaction at all! Instead it is a physical reaction. That means that all of the pieces of the reaction are there, but that they are simply re-arranged.

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